The invention relates to a measuring device having a protective cover and a sensor arranged in the protective cover.
In principle, such measuring devices are known, e.g., from Japanese patent application publication JP 57-101730 or from U.S. Pat. No. 4,124,465. These measuring devices are used for temperature measurements in aluminum melts or cryolite melts. Basically, for measurements in such melts or in other aggressive high-temperature melts, there is the problem that the sensor itself can be attacked by the aggressive melt and must be protected adequately against this melt. In particular, such protection is important for measuring devices that are used for long-term measurement. For such measuring devices it is to be ensured, on the one hand, that the sensor itself comes into adequate contact with the melt to be measured, but also, on other hand, that it is adequately protected.
For temperature sensors as thermocouples, for example, this means that good heat contact must be ensured between the sensor and the melt for simultaneous protection from chemical corrosion, from electrochemical corrosion, and against mechanical damage, which could be caused by temperature shocks when the measuring device is immersed in the melt. Sensors that are designed for use in molten aluminum or molten cryolite must be protected against electrochemical corrosion, which is otherwise generated in the bath by the electric field. Furthermore, cryolite is a relatively chemically aggressive material. The aluminum melt has a strongly reducing effect, so that the measuring device must be protected against reduction. At the same time, protection against oxidation is required, because, for example, the carbon dioxide that is also normally present promotes oxidation of the measuring device.
Such protection must at the same time allow sufficient accuracy of the measurement. For the production of aluminum by electrolysis, the electrodes or the electrolysis cell itself is often protected by titanium boride, as described, for example, in European Patent EP 0 973 955 B1. Additional materials for the protection of electrolysis cells are disclosed in U.S. Pat. Nos. 6,402,926 B1 or 4,560,448. Also, International patent application publication No. WO 2007/110148 A1 describes anti-corrosion protection layers for thermocouple protective tubes, which are used in cryolite melts. Here, a multiphase material, whose main component can be titanium boride, is described.
In practice, it has been proven that coatings made of the known materials do not adequately protect a sensor, in particular a temperature sensor, in order to allow a measurement over a long period, so that the measurement device could be used reliably beyond a single temperature measurement.